Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 5

Big Idea #3: Chemical Reactions, Energy Changes, and Redox Reactions

ENTHALPY

Enthalpy Change, H

The enthalpy of a substance is a measure of the energy that is released or absorbed by the substance when bonds are broken and formed during a reaction.

The Basic Rules of Enthalpy

When bonds are formed, energy is released.

When bonds are broken, energy is absorbed.

The change in enthalpy, ∆H, that takes place over the course of a reaction can be calculated by subtracting the enthalpy of the reactants from the enthalpy of the products.

Enthalpy Change

H = Hproducts − Hreactants

If the products have stronger bonds than the reactants, then the products have lower enthalpy than the reactants and are more stable; in this case, energy is released by the reaction, or the reaction is exothermic.

All substances are the
most stable in their
lowest energy state.
This means that, in
general, exothermic
processes are more
likely to be favorable
than endothermic
processes.

If the products have weaker bonds than the reactants, then the products have higher enthalpy than the reactants and are less stable; in this case, energy is absorbed by the reaction, or the reaction is endothermic.